This application claims the priority of German Patent Document 199 22 612.1, filed May 17, 1999.
The invention relates to a micromechanical pump, with a peristaltic actuator for transporting and/or managing defined quantities of liquid or gas (pump medium).
The metering of minimum quantities of liquid in the microliter up to the nanoliter range is becoming ever more relevant for many applications in analytic chemistry and medical or environmental engineering. Frequently it is of interest to receive a defined quantity of liquid at one place, to transport it and deliver it at another place. Tasks of this nature are an essential part of quantitative analysis. Modern equipment can meter a few tens of microliters up to a few hundred microliters of a liquid with an accuracy of better than one percent by means of stepping motor-controlled injection pumps and precision pipettes. To manipulate, however, quantities of a few hundred nanoliters up to a few tens of microliters with the same accuracy, it is necessary to find other concepts of metering.
The micropump-based metering systems are dominated by two concepts. On the one hand, membrane pumps with two passive valves are used, and, on the other hand, valveless pumps, according to the diffuser nozzle principle, which are not sealed in the idle state, are used. Both types are unidirectional. That is, they can transport only in one direction. In both cases piezo actuators, which are cemented on the pump membrane, are usually used as the drive. An electrostatically driven micromembrane pump with passive valves is known from the German Patent DE 19719862. At high drive frequencies, this pump""s direction of delivery reverses itself owing to the inertia of the passive valves. However, this property can be used only to a limited degree for pumping backwards. The delivery rate is a function not only of the supplied power, but also of the properties of the pump medium to be pumped. Therefore, it is not possible to conclude from the supplied electric power the flow rate of an arbitrary pump medium. Since the volume displaced at each stroke of the pump is only a fraction of the volume of the pump chamber, the pump has a high dead volume.
The U.S. Pat. No. 57 05 018 discloses a micromechanical peristaltic pump, where the medium to be pumped is delivered by means of an electrically conductive membrane in a cavity equipped with electrodes. This pump has the drawback that it does not seal in the idle state and that the voltage between membrane and electrodes decreases across the medium to be pumped. Pumps with a circular drive element are disclosed, for example, in the WO 98/07199.
The principle of a pneumatic coupling of entrapped air volumes for a microvalve is known from the published German patent application DE 196 37 928 A1, where a micropump system, based on this principle, is disclosed. The drawback of this system is that the membrane does not form a tight seal at the cover. Hence, additional valves are needed to guarantee that the pump is tight in the idle state. Furthermore, since the delivery channel does not exhibit an endlessly continuous shape, the pump medium cannot be transported continuously without interruption. These drawbacks have a negative effect on the accuracy of the metering capability and on the output of the pump.
In summary one can state that there exist no metering systems that receive a predetermined quantity of liquid at one place and deliver it again at another place. Micropumps, which can meter precisely in the range below ten microliters, are also not available.
The invention is based on the problem of providing a bidirectional micropump, which can both transport in the microliter range continuously and manage defined volumes of liquid.
The micromechanical pump, based on the principle of a peristaltic actuator, is formed by stretching sealingly an electrically conductive membrane over a preferably ring-shaped hollow space, which continues endlessly in the linear direction and is filled with a drive medium, in a substrate (cavity). Electrodes, which can be driven separately at least to some degree, are installed permanently on the floor of the cavity. When some of the electrodes are driven, the membrane above the driven electrodes is pulled downwards; and through displacement of the drive medium the membrane above the non-driven electrodes is pressed upwards.
In this respect the electrodes have to be separated from the membrane with a passivating layer so that in the driven state no short circuit can occur between the electrodes and the membrane. This layer is applied preferably on the underside of the membrane and in the case of a silicon membrane is made preferably of a silicon oxide.
Because the cavity is closed, i.e. forms a hollow space with a fixed volume, and because this cavity contains the drive medium, the membrane must bulge upwards, owing to the displacement of the drive medium out of the areas of the cavity, in which the membrane is pulled downwards, to locations where the electrodes are not driven. If an adequate number of electrodes are driven, the drive medium is compressed below the upwardly bulging membrane areas in such a manner that the buckled areas are pressed firmly against the cover. This effect is called pneumatic coupling. While the electrodes are driven in pairs by a suitable method next to the buckled areas, the buckled areas can be slid over the cavity. The actuation of the electrodes is switched off in the pumping direction behind the buckling and switched on in front. Thus, it involves an indirect drive. The pump medium is displaced in the pumping direction not directly by the actuation of the electrodes, but by the displacement of one or several of the buckled area(s), under which the drive medium is compressed. The drive medium can be a liquid or a gas. If it is a liquid, then the volume of the liquid quantity must be less than the volume of the cavity. Owing to the incompressibility of liquids, the membrane cannot bulge downwards. Owing to the smaller volume of liquid, the membrane is bulged downwards already in the unactuated state. The results are analogous when, in the case of a gas as the drive medium, there is a vacuum in the hollow space of the cavity covered by the membrane.
It is especially advantageous, in the case of a vacuum of the drive medium or in the case of a liquid as the drive medium, if the membrane is under compressive stress, so that it forms a buckled area by itself (spontaneous buckling) without having to drive the electrodes. The spontaneous buckling can be achieved, for example, with a silicon membrane by oxidizing on a layer of silicon oxide. Such a membrane has the property that it bulges downwards and in other areas upwards in some areas over the cavity. The pump, according to the invention, holds its state, assumed last, by actuating the electrodes, even when the actuation of the electrodes is switched off. In the unpowered, i.e. non-driven state, the pump medium cannot flow through the pump in the case of spontaneous buckling of the membrane. In the ideal case the spontaneous buckling is characterized in conjunction with the vacuum or the fixed volume of liquid in the cavity in such a manner that the result is a desired number of seals, which are just adequately wide to cover the inlets and outlets of the pump.
To build a pump, the drive element, i.e. the membrane, is tightly covered with a flat cover, containing the inlet and the outlet. In the areas of the driven electrodes, where the membrane is pulled downwards, the result is a gap between the cover and the membrane, whereas in the areas of the, non-driven electrodes the membrane is pressed against the cover. This gap serves to receive the pump medium. When the electrodes are driven selectively, the area, in which the membrane is pressed against the cover, can be moved peristaltically from the inlet to the outlet. The pump medium, enclosed in the gap, is transported so as to be defined. However, the cover can also exhibit a notched surface above the cavity that is similar to the cavity, thus a passage. Due to the high forces pressing the membrane against the cover due to pneumatic coupling in the buckled membrane areas, the buckled area can adapt in accordance with the elasticity of the membrane to the shape of the passage.
Especially advantageous is the preferably annular shape of the cavity which forms an endlessly continuous channel for a drive medium such that the cross sectional shape of the cavity can match the shape of the downwards buckled membrane and the shape of the electrode is also curved in such a manner. In this case even if the membrane rests tightly against the substrate in the driven state and the drive medium can be displaced, therefore, only in the pumping direction, a continuous pumping process can be maintained. This would not be possible in the case of a cavity that were not endlessly continuous, since then the electrodes would have to be driven in such a manner that the drive medium for the next pump cycle would have to be displaced again in the direction of the inlet.
The cross sectional shape of the cavity that is adapted to the downwards buckled membrane is especially advantageous, if the drive medium is a gas, because then, as the electrodes are driven, the pressure increase in the gas, due to its lower volume, is especially high; and the membrane in the unselected areas is pressed tighter against the cover; and, therefore, the pump seals better. This feature is especially advantageous when used as a valve.
The pump is bidirectional. That is, there is the possibility of reversing at any time the pumping direction; and the pump has a low dead volume, a feature that is of decisive importance for pumps with compressible mediums, i.e., gases. The suction and the pump pressure of the pump, according to the invention, are approximately identical, since the shape of the bulges produced by the compressed gas or the fixed volume of the drive medium liquid is determined on all sides by means of the driven electrodes. The drive can occur separately from the medium. That is, the electrodes do not make contact with the pump medium. In particular, the voltage between the driven electrodes and the membrane must not drop directly across the pump medium.
Because the capacity between individual electrodes and the membrane in the arched areas differs considerably from the capacity in the areas adjoining below, the pumping process can be monitored electronically. Controlled metering of the smallest quantity of liquid is possible.
Another advantage is that the membrane is under compressive stress, that is, bulges in sub-areas without any actuation (spontaneous bulging), so that because it is less necessary for the membrane to expand, less force has to be expended to move the arched areas, serving as a seal, inside the cavity. When the membrane is under compressive stress, less force is needed to form the defined bulges, since sub-areas of the membrane are already bulged downwards and thus, in these areas, no force has to be generated to stretch the membrane in the direction of the cavity. Then the pump can be driven with significantly less voltage; and the result is a bistable valve, whose switching states can be held without power.
The present invention is described below with reference to the embodiments and the drawings without restricting the general inventive idea.